In laboratory or manufacturing buildings using a large number of fume hoods connected to a central exhaust system of essentially constant flow volume, the system should be balanced by adjusting the volume of air exhausted through each hood to within the permissible air velocity limits. In order to keep the system in balance, it is necessary that the volume of air supplied from each hood be maintained reasonably constant, whether the hood is in use or not and, in the case of enclosed hoods with access windows, whether the window of the hood is open or closed.
As a result, during periods when the hood is not in use, tempered and often purified room air is still being exhausted in large volume since the connection of the hood to the exhaust system cannot be closed off without upsetting the balance of the system. Similarly, when the hood is in use, the volume of air drawn through it must be maintained at the high value required to sustain the requisite air velocity through an open access window and cannot be significantly reduced when the window is closed. This results in a substantial waster of the energy required to heat or cool the room air which is then uselessely discharged through the exhaust outlet in great volume.